A technique for imaging a transmission intensity distribution using the material transmission performance of radiation represented by X-rays is a fundamental of modern medical techniques. Since the discovery of X-rays, imaging of the intensity distribution of radiation has adopted a method in which an X-ray intensity distribution is converted into visible light by a phosphor, a latent image is formed by using a silver halide film, and the latent image is developed. In recent years, a method of using a so-called imaging plate has been generally used. This method uses a photostimulable phosphor upon converting an X-ray image into a digital image, and reads out a latent image as an accumulated energy distribution on the photostimulable phosphor by exciting it by a laser beam, thus capturing a digital image. Furthermore, with the advance of semiconductor technologies, a large-sized solid-state image sensing element that can cover the human body size, i.e., a so-called flat panel detector, has been developed, and an X-ray image can be directly converted into a digital image without forming any latent image. In this way, efficient diagnosis can be made.
On the other hand, the behavior of the interior of the human body can itself be observed by imaging fluorescence of weak X-rays using a high-sensitivity image sensing element represented by an image intensifier, and such method is popularly used. The latest flat panel detector has a sensitivity as high as the image intensifier, and allows one to radiograph the behavior of the human body over a broad range.
Chest radiography of the human body is most effective in medical radiography. Since radiographed images over the broad range of the chest, including the abdomen, serve to find many diseases, including lung disease, chest radiography is indispensable in normal health examinations. In recent years, in order efficiently to diagnose chest X-ray images in huge quantities radiographed for the health examination, so-called Computer-Aided Diagnosis (CAD) for applying image analysis to digital chest X-ray images using a computer to help initial diagnosis by a doctor has been put into practical use.
As an example of image analysis, a paper by G. F. Powell, K. Doi, and S. Katsuragawa, “Location of Inter-Rib Spaces for Lung Texture Analysis and Computer-Aided Diagnosis in Digital Chest Images”, in Med. Phys. 15, 581-587, 1988 (to be referred to as reference 1 hereinafter), describes a technique for extracting a lung field part from a digital chest X-ray mage.
When a suspicion of a disease of some kind is raised by the health examination, a definite diagnosis based on q so-called diagnostic workup is made. Such definite diagnosis normally involves a diagnostic cost several times that of normal radiography, such as a CT/MR scan or the like. Furthermore, the diagnostic workup may often determine that the initial diagnosis is a false diagnosis, and no disease is found. Hence, wasteful use of medical resources has been incurred; this represents part of the reason for medical cost inflation.
In order to avoid such problem, the accuracy of the health examination—the initial diagnosis—must be improved. As a method of improving accuracy while suppressing a rise in cost, it is effective to perform behavior observation by capturing a chest moving image representing the body's behavior in respiration and the like using the aforementioned large-sized flat panel detector. In order to assure stable behavior observation, the patient must breathe in an appropriate fashion. Therefore, imaging must be started while the patient appropriately respires, but it is difficult in practice to recognize and use the correct timing. An apparatus that monitors respiration is available. However, that apparatus is high in cost and involves complicated operations for the patient.